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—vsässr- (INTERIM REPORT) C0PV NR -

SCIM -A METER FOR MEASURING THE PERFORMANCE

OF SPEECH COMMUNICATION SYSTEMS

TECHNICAL DOCUMENTARY REPORT NO. ESD-TDR-64-674

COPIES

OCTOBER 1964

Kar! D. Kryter Jay H. Ball

figTI PROCESSED

D DDC TAB D PROJ OFFICER

D ACCESSJON MASTER FILE

D

DATE,

ESTI CONTROL NR. AL 45135

CY NR. OF- i- -era

« e~?£ SCIENCES LABORATORY ELECTRONIC SYSTEMS DIVISION AIR FORCE SYSTEMS COMMAND

UNITED STATES AIR FORCE L.G. Hanscom Field, Bedford, Massachusetts

Project 2808, Task 280802

(Prepared under Contract No. AF 19 (MR\ OAAD L » . » Cambridge, Massachusetts.? <628)"2^3 by Bolt Beranek and Newman Inc.,

•ßUkKbf^

DDC AVAILABILITY

Qualified requesters may obtain copies from Defense Documentation Center (DDC). Orders will be expedited If placed through the librarian or other person designated to request docu- ments from DDC*

DISSEMINATION NOTICE

Copies available at Office of Technical Services, Department of Commerce«

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When US Government drawings, specifications or other data are used for any purpose other than a definitely related government procurement operation, the government thereby incurs no responsibility nor any obligation whatsoever; and the fact that the government may have formulated, furnished, or in any way supplied the said draw- ings, specifications, or other data is not to be regarded by implication or otherwise as in any manner licensing the holder or any other person or conveying any rights or permission to manufacture, use, or sell any patented invention that may in any way be related thereta. QTHER ^^

Do not return this copy. Retain or destroy.

ESD-TDR-64-674

(INTERIM REPORT)

SCIM — A METER FOR MEASURING THE PERFORMANCE

OF SPEECH COMMUNICATION SYSTEMS

TECHNICAL DOCUMENTARY REPORT NO. ESD-TDR-64-674

OCTOBER 1964

Karl D. Kryter Jay H. Ball

DECISION SCIENCES LABORATORY ELECTRONIC SYSTEMS DIVISION AIR FORCE SYSTEMS COMMAND

UNITED STATES AIR FORCE L. G. Hanscom Field, Bedford, Massachusetts

Project 2808, Task 280802

(Prepared under Contract No. AF 19 (628)-2463 by Bolt Beranek and Newman Inc. , Cambridge, Massachusetts.)

FOREWORD

This technical report presents the results of a study performed by Bolt, Beranek and Newman, Inc., Cambridge, Massachusetts, on Air Force Contract AF 19(628)-2U63- This contract was initiated under Project 2808, "Psychoacoustic Standards in Voice Communication System Evaluation," Task 28o802. This task was monitored under the direction of the Decision Sciences Laboratory, Mr. Stephen E. Stuntz, ESRHA.

ESD-TDR-64-674

TABLE OP CONTENTS

Page

SECTION 1 INTRODUCTION 1

SECTION 2 GENERAL RELATIONS BETWEEN INTELLIGIBILITY AND PHYSICAL CHARACTERISTICS OP SPEECH 3

SECTION 3 SCIM 15

SECTION 4 TEST RESULTS 28

SECTION 5 FUTURE DEVELOPMENTS 34

REFERENCES 36

DOCUMENT CONTROL DATA - R+D 37

ILLUSTRATIONS

Fig, 1 Showing Four Curves Relating Different Measures of Speech Level to Frequency (after Dunn and White?) 5

2 Threshold Shift Due to a 350-450 cps Masking Noise at 85 dB Overall Sound Pressure Level (after Carter and Kryter°) 6

Work Sheet for the Calculation of the Articu- lation Index (Al). Speech Spectrum Plotted is the Long-Term RMS of Speech Plus 12 dB, the Dashed Curve is the Spread-of-Masking to be Expected from the Octave Band of Noise that is Plotted on the Curve (see Ref. 2) 8

Showing the Increase in RMS Speech Power as a Function of Clipping when Clipped Level is Raised to Clipping Reference Level (after Wathen-Dunn and Lipke?) 10

Comparison of Obtained and Predicted Test Scores for Broad-Band Speech in the Presence of Narrow Bands of Noise Set at Various Intensity Levels (after Miller8) 12

ESD-TDR-64-674

TABLE OP CONTENTS (cont'd) Page

Pig. 6 Comparison of Obtained and Predicted Test Scores for Speech Passed through a Bandpass Filter and Heard in the Presence of Broad- Band Negatively Sloped Spectrum Noise Set at Various Intensity Levels (after Egan and Wiener9) 13

7 Relation between AI and Various Measures of Speech Intelligibility (after French and Steinberg1 and Miller0) 14

8 Photo of Signal Generator 16

9 Simplified Block Diagram of Signal Generator. 17

10 Test Signal Sequence 18

11 Photo of Analyzer 20

12 Simplified Block Diagram of Analyzer 21

13 SCI Multiplier vs. Frequency Shift 27

14 Showing the Maximum, Minimum and Average Values of the Deviation of Five Measured SCI's from the Calculated AI*s at S/N Ratios from -10 dB to +22 dB 29

15 Showing Percent MR Test Scores as a Function of SCI for Laboratory Speech Systems, AF H-F SSB, and Microwave Radio Systems (see Table 1) 32

SCIM -- A METER FOR MEASURING THE PERFORMANCE

OF SPEECH COMMUNICATION SYSTEMS

ABSTRACT

Two major theories have emerged that attempt to relate in a

quantitative way the physical characteristics of a speech com-

munication system and the intelligibility of speech as perceived

by a crew of trained listeners.

The more widely used theory is the basis for the "Articulation

Index" (AI) formulated in 19^7 by N. R. French and J. C. Stein-

berg of the Bell Telephone Laboratories. This theory holds

that the signal (speech)-to-noise ratio (in 20 narrow frequency

bands) of a communication system will, when properly weighted

and summed, provide an AI value that is directly related to the

intelligibility of speech heard over that system.

A second theoretical approach to this problem is embodied in the

Pattern Correspondence Index (PCI) machine proposed by J.C.R.

Licklider in 1956. This instrument determines the correlation

in the frequency and time domains between the speech input to a

communication system and the speech output of that system.

The subject of the present paper is an electronic device called

SCIM (Speech Communication Index Meter) that is designed primarily

in terms of the Articulation Index theory; however, SCIM accom-

modates additional factors affecting speech that are not involved

in the original procedures for calculating AI. The SCIM signal

generator generates a 3-second signal burst, which is fed to the

electrical input of the system under test; within 12 seconds the

SCIM analyzer, located at the receiving station, calculates and

displays an index, ranging from .00 to .99* which reflects the

ability of that system to transmit intelligible speech.

iii

Although devices based on similar principles have been previously

developed, SCIM measures several parameters affecting the per-

formance of a speech system differently than previous instruments.

SCIM was designed to be relatively small, have small power re-

quirements, be capable of remote digital readout, and contain

other features to make it suitable for field, fixed station or

laboratory use.

The results of SCIM measurements and actual speech intelligibility

tests obtained from a variety of communication systems and

operating conditions will be presented.

REVIEW AND APPROVAL

This tec pport has been reviewed^nd is approved.

/ JOS^T^BEidS^'^T^ ROY MORGAN P Chief, Applications division Colonel, USAF Decision Sciences Laboratory

uoionei, ubAP Director, Decision Sciences Laboratory

iv

SECTION 1

INTRODUCTION

The valid assessment of the performance of speech communica-

tion systems has always been a difficult problem. The re-

quirements for assessment have been, usually, two: (l) as

aid and guidance in the design of electronic components

(microphones, amplifiers, transmitters, receivers, earphones,

etc.) and (2) the objective monitoring of the performance of

communication systems during actual operations or under

simulated operational conditions.

The task, of course, has been made difficult because, physi-

cally, the speech has such complex and dynamic characteristics,

and secondly because the information content of the speech

signal, as perceived by the human listener, is not always

simply related to the physical nature of the speech signal,

at least insofar as our present methods of physical analysis

would indicate.

Speech intelligibility or speech articulation tests provide,

when properly used, a reasonably accurate and valid procedure

for measuring differences in the performance of communications

equipment and systems. However, these psychological tests,

which usually require laboratory conditions for their admin-

istration and specially trained crews of talkers and listeners,

are costly and time-consuming, and, in most cases, cannot be

readily applied to the measurement of speech systems under

"real life" operational conditions.

Nevertheless, from 30 odd years of laboratory research with

speech intelligibility and articulation tests, there has

-1-

emerged a rather extensive set of facts about the relation

between measurable physical characteristics of the received

speech signal, including the noise, if any, present with the

speech and the understandability of speech as perceived by

listeners. Various investigators have attempted to use this

knowledge to develop methods of system evaluation that avoid

the speech testing process per se, but yet provide a measure

that is interpretable in terms of speech understandability.

One outcome of this latter effort has been the development

of methods for calculating from physical measurements of a

communication system what the intelligibility of speech

should be over that system; a secondary outcome has been the

design of electronic test devices which, when applied to a

speech communication system, automatically evaluate the per-

formance of that speech system.

The purposes of the present paper are: (l) to outline briefly

the theory and scientific basis for the design of one such

test instrument (called the Speech Communication Index Meter,

"SCIM") for measuring the performance of speech communication

systems; (2) a description of the SCIM device; and (3) a

comparison of the measurements achieved by the SCIM instrument

for a variety of speech communication systems with the results

of speech intelligibility tests obtained for the same systems.

-2-

SECTION 2

GENERAL RELATIONS BETWEEN INTELLIGIBILITY AND PHYSICAL

CHARACTERISTICS OF SPEECH

Two major theories have emerged that attempt to relate in a

quantitative way the physical characteristics of a speech

communication system and the intelligibility of speech as

perceived by a crew of trained listeners.

The more widely used theory is the basis for the "Articulation

Index" (Al) formulated in 19^7 by N. R. French and J. C.

Steinberg of the Bell Telephone Laboratories. This theory

holds that the speech-to-noise ratio, in 20 narrow frequency

bands, of a communication system will provide an AI value

that is directly related to the intelligibility of speech

heard over that system.

A second theoretical approach to this problem is embodied

in the Pattern Correspondence Index (PCI) machine proposed

by J. C. R. Licklider in 1956. This instrument determines

the correlation in the frequency and time domains between

the speech input to a communication system and the speech

output of that system.

SCIM is designed primarily in terms of the Articulation Index

theory; however, SCIM accommodates additional factors affect-

ing speech that are not Involved in the original procedures

for calculating AI.

-3-

In order to explain the Articulation Index, it is necessary

to describe some of the general characteristics of the speech

signal and of masking by noise.

Figure 1 shows the variations in the pressure of conversa-

tional speech measured over short, l/8-second intervals of

time (l/8 second was chosen because it represents the average

duration of individual speech sounds) as a function of fre-

quency. The graph is plotted in terms of the pressure per

cycle; we see that speech contains energy over the frequency

range from at least 100 to 7000 cps and that the range of

pressures is in excess of 30 dB over this entire frequency

range. This range of pressures is the range found among

individual speech sounds when the speech is uttered at a

supposedly normal constant level of effort. It does not

represent the range between weak talking or shouting, or

amongst different talkers. This figure demonstrates the

complex and dynamic nature of the speech signal.

Masking. A factor which enters into the calculation of AI

is the masking effect of noise; noise containing sufficient

energy, at a given point on the frequency scale, can mask or

prevent the reception of speech components not only in the

same frequency region, but also frequency components of

speech that lie above and below the frequency components of

the noise. Upward spread-of-masking, in which a low-frequency

tone or noise masks a higher frequency portion of the speech

spectrum, is far more severe than downward spread-of-masking,

in which the masking tone is higher in frequency than the

signal of interest. Figure 2 illustrates the effect of this ?T spread-of-masking" phenomenon upon the perception of pure

tones.

-4-

Ul Q_

100

*

200

Re 0.0002Mbor

500 1000 2000 FREQUENCY(cps)

5000

FIG.l SHOWING FOUR CURVES RELATING DIFFERENT MEASURES OF SPEECH LEVEL TO FREQUENCY (AFTER DUNN AND WHITE5)

Peak instantaneous pressures

\- Speech peaks

Long-time rms pressures

J—Speech minima

10000

00

80

CD Q 60

I

i

X If)

o X if) LU or X

40

20

0

-20

c u i CT rvi ir Tr*

NARROW BAND NOISE

THRESHOLD SHIFT DUE TO WHITE NOISE FLOOR

SENSATION LEVEL OF THE MASKING NOISE

7 ^

/

/

/

I

i /^ \

i «^-

./

\ . «

f i

x^

V J x w^

20 56789 2 3 456789

100 1000 FREQUENCY IN CYCLES PER SECOND

5 6 7 8 9 10,000

FIG.2 THRESHOLD SHIFT DUE TO A 350-450 CPS MASKING NOISE AT 85 DB OVERALL SOUND PRESSURE LEVEL (AFTER CARTER AND KRYTER6)

Figure 2 shows that a narrow band of noise centered at 400 cps

would mask information around 800 cps as effectively as a

band of noise centered at 800 cps but of 18 dB less amplitude

than the band centered at 400 cps; it would also mask infor-

mation at l600 cps to much the same degree as would a -30 dB

band of noise located at l600 cps.

Calculation of the Articulation Index. As previously mentioned,

the AI of a speech communications link is normally calculated

by measuring the speech-to-noise (S/N) ratio in a number of

frequency bands; the AI is defined as the weighted sum of

these S/N ratios. There are two fundamental restrictions

placed upon the calculated AI -- namely, that it can never be

less than 0.00 and that it may never exceed 1.00. To implement

the former restriction, we assume that all measured S/N ratios

less than -12 dB are equal to -12 dB; and, since the band-

weighting functions are derived so that the AI equals 1.00

when the S/N ratio in all bands is +18 dB, we assume that all

S/N ratios in excess of +18 dB are equal to +18 dB. Figure 3

illustrates the general concept and work sheet utilized for

the calculation of AI. In Fig. 3 the speech spectrum is

divided into 20 narrow bands; these bands have been chosen

so that each band contributes equally to speech intelligibility.

Also, it is seen that speech at very weak and very intense

levels does not contribute to intelligibility because it ex-

ceeds the limits of the normal ear.

On Fig. 3, we have sketched the speech spectrum that might be

present at the listener's ear from a typical speech system

along with an octave band of masking noise that is also pre-

sumed to be mixed with the speech at the listener's ear. Note

that the masking spectrum, due to the aforementioned upward

-7-

I 00

100 200

*Re 0.0002 ^bar

500 1000 FREQUENCY (cps)

2000 5000 10000

FIG.3 WORK SHEET FOR THE CALCULATION OF THE ARTICULATION INDEX (Al). SPEECH SPECTRUM PLOTTED IS THE LONG-TERM RMS OF SPEECH PLUS 12DB, THE DASHED CURVE IS THE SPREAD-OF-MASKING TO BE EXPECTED FROM

ittE lfAüiB^ 8tN(iUE iUiAT«L? [iUT14ß cWHUu&tf ÄE ßtf-JL

spread-of-masking of the noise exceeds, at the higher fre- 2

quencies the actual noise spectrum. The AI is calculated

by taking the S/N ratios between the plotted speech

spectrum (long-term rms plus 12 dB) and the noise or mask-

ing spectrum, whichever is the greater for each of the

20 bands, weighting proportionately each S/N ratio so that

+30 dB equals 1.00 and 0 dB equals 0, summing the result

and dividing by 20.

Frequency shift. The AI must be corrected for factors other

than noise that affect intelligibility. One such factor is

frequency shift. If a single sideband ratio receiver is not

tuned precisely to the carrier frequency of the transmitter,

the detected speech signal will appear shifted upwards or

downwards by an amount equal to the mismatch between the

transmitted carrier frequency and the receiver local oscil-

lator frequency. While upward shifts in the speech spectrum

of 100 cps or more can be tolerated, a downward shift of

50 cps can have a devastating effect upon the intelligibility

of the speech.

The correction factor to be applied because of the presence of

frequency shift will be shown in a later figure.

Peak clipping. Peak clipping is still another factor which

enters into the calculation of AI. Figure 4 shows the in-

crease (over undipped speech of the same peak amplitude) in

the long-term rms pressure of peak-clipped speech as a func-

tion of the amount of clipping and subsequent reamplification

-9-

AMOUNT OF PEAK-CLIPPING AND POST-CLIPPING AMPLIFICATION IN DB (PEAK AMPLITUDE DEFINED BY O.OOI PROBABILITY LEVEL IN DISTRIBUTION OF INSTANTANEOUS AMPLITUDES)

FIG.4 SHOWING THE INCREASE IN RMS SPEECH POWER AS A FUNCTION OF CLIPPING WHEN CLIPPED LEVEL IS RAISED TO CLIPPING REFERENCE LEVEL (AFTER WATHEN-DUNN AND LIPKE7)

-10-

required to restore the undipped peak levels. Peak clipping

must be taken into account when calculating AI because the intel-

ligibility of speech is, to a first approximation, proportional

to the long-term rms signal-to-noise ratio and not to the peak

S/N ratio.

Validity of AI. The validity of the AI concept has been well

established for a wide variety of speech communication systems.

Some of the results of the application of the AI calculation

procedure to speech systems for which speech intelligibility

test scores were available are presented in Pigs. 5 and 6.

Figure 7 shows the relation between AI and different types of

speech test materials. It is to be noted that the percentage

of test items correctly perceived is dependent not only on the

speech material -- nonsense syllables, words, or sentences -- but also the size of the message set, whether, for example, the

test vocabulary consisted of a 1000, 256, or 32 "PB" (so-called

phonetically balanced) words.

-11-

0.4 0.6 ARTICULATION INDEX

FIG.5 COMPARISON OF OBTAINED AND PREDICTED TEST SCORES FOR BROAD-BAND SPEECH IN THE PRESENCE OF NARROW BANDS OF NOISE SET AT VARIOUS INTENSITY LEVELS (AFTER MILLER8)

-12-

100

o Lü tr £80 o CO LLJ

CD <

>- CO

LU CO

UJ CO

60

40

UJ u LU CL

20

^m\

A fiX

>x*^"

• ■ ■

X

/l

7 NOISE SPECTRUM "B"

FILTER PASS BAND

NO. i • 130-9200 CPS N0.2 x 340-3900 NO. 3 O 550-2500

F ^

^

NO. 5

1

V

550-

1 1

1500

1 I 1 0 0.2 0.4 0.6

ARTICULATION INDEX 0.8 1.0

FIG.6 COMPARISON OF OBTAINED AND PREDICTED TEST SCORES FOR SPEECH PASSED THROUGH A BANDPASS FILTER AND HEARD IN THE PRESENCE OF A BROAD-BAND NEGATIVELY SLOPED SPECTRUM NOISE SET AT VARIOUS INTENSITY LEVELS (AFTER EGAN AND WIENER9)

-13-

100

o er Lü er

i

Q O o 00 er Lü Q 70

90

80

CO LÜ CJ>

LU co

er o CO Q er o

60

50

40

^30 CO LU _J CD

« 20

>- CO

10

0

-TEST VOCABULARY LIMITED T0 32 PB WORDS

SENTE NCES-

\ PB WORDS (1000 DIFFERENT WORDS)

MOM-^FM^F ^Yl 1 ARI FQ IN U IM JLINOL J ILLMDLLJ

(1000 DIFFERENT SYLLABLES)

\

-TEST VOCABULARY LIMITED T0 256 PB WORDS

NOTE: THESE RELATIONS ARE APPROXIMATE, THEY DEPEND UPON Skll 1 OF TAI KFRS AND

LISTEN ERS

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 ARTICULATION INDEX

0.8 0.9 1.0

FIG.7 RELATION BETWEEN Al AND VARIOUS MEASURES OF SPEECH INTELLIGIBILITY (AFTER FRENCH AND STEINBERG1 AND MILLER«)

-14-

SECTION 3

SCIM

Although the AI procedure is objective, it requires tedious

calculations from physical data about speech systems that are

not always easily obtained.

The Speech Communication Index Meter is an electronic device

which automatically calculates, albeit in a somewhat simplified

fashion, the AI of any given speech system. There has been at

least one previous instrument built on similar principles.

General Electronics Laboratories Inc. of Cambridge, Massachu-

setts, designed and built for the U. S. Army an instrument

called the Voice Interference Analysis Set (VIAS) which also

estimates the AI of speech systems. VIAS differs, however, in

a number of respects from SCIM, both in the type of signals

utilized for system testing and in the processing and analysis

of these signals.

Signal generator. The SCIM signal generator, shown in Figs.

8 and 9, located at the transmitting terminal of the link

under test normally presents for transmission a continuous

1 kc calibration tone. When the START button is depressed,

the signal generator synchronizes itself, resulting in a maxi-

mum delay of one second before the beginning of the test

sequence shown in Fig. 10.

The sequence begins with the replacement of the 1000 cps cali-

bration tone by a 30-millisecond sync burst. The burst consists

of the sum of a 600 cps sine wave and a 2000 cps sine wave; the

function of the burst is, of course, to signal the analyzer that

the test sequence has begun.

Following the sync burst there appears a "speech" signal, pro-

duced by square-wave modulating a shaped-spectrum random noise

-15-

i

FIG. 8 PHOTO OF SIGNAL GENERATOR

I

NOISE SOURCE

SPECTRUM SHAPING FILTER

SYNC. GENERATOR

MODULATOR

3 KC OSCILLATOR

I KC OSCILLATOR

■O 6

7L

400 CPS OSCILLATOR

SIGNAL OUT TO SYSTEM UNDER TEST

FIG.9 SIMPLIFIED BLOCK DIAGRAM OF SIGNAL GENERATOR

^ww /WWVWWWWl ©

1 KC CAL TONE

i H co

MODULATED NOISE

3 KC AGC TONE 400 CPS AGC TONE

1

© 1

' * r • * 1 ® 1© 1© dJ

i ©

JjL iiiiiii

pr P» 'i ' i

1 ■ 1

COMPLETE SEQUENCE

TONE PAIR (TYP.)

FIG.10 TEST SIGNAL SEQUENCE

source. Prior to modulation the noise is tailored, by means of

a spectrum-shaping filter, to have an amplitude-versus-frequency

characteristic which closely approximates the long-term root-

mean-square spectrum of speech. Modulation is performed at a

5 cps rate by means of a time-varying attenuator which provides

an on-to-off attenuation ratio of 17 dB. Since the modulation

duty cycle is 50%, the high-level and low-level portions of the

"speech" signal each last for 1/10 second.

At the end of one second, the "speech" signal transmission is

terminated and a 3000 cps tone appears instead. A 30-millisecond

sync burst is used to mark the temporal boundary between the

"speech" and the tone. While the 3000 cps tone is on, measure-

ments are made by the analyzer of the background noise, in the

frequency range 250-1650 cps, of the communication system under

test; the tone serves to keep constant the gain of any AGC

equipment which may be part of the system being tested. After

one second has elapsed, the 3000 cps tone is replaced by a

400 cps tone, which serves the same purpose but allows system

noise measurements to be made in the 1650-4200 cps portion of

the spectrum.

As before, a 30-millisecond sync burst delimits the end and

beginning, respectively, of the 3000 and 400 cps AGC tones.

At the end of this third one-second interval a sync burst is

presented and the signal generator output reverts to the

1000 cps calibration tone, which will continue until the next

time that the START button is depressed.

Analyzer. Located at the receiving terminal, the SCIM analyzer

(shown in Figs. 11 and 12) filters the received "speech" or

noise spectrum into nine bands, which together cover the fre-

quency range from 250 cps to 4200 cps. The lower-skirt slope

of all filters is +18 dB/octave, the upper-skirt -60 or more

-19-

FIG.11 PHOTO OF ANALYZER

-20-

RECEIVED SIGNAL IN FROM SYSTEM UNDER TEST

I r\3

CLOCK

SYNC. DETECTOR

INTEGRATOR CONTROL

BANDPASS FILTERS AND INTEGRATORS DETECTORS

NO.l

TO 'INTEGRATORS

TO INTEGRATORS

COMMUTATOR CONTROL

u

]-l is

^H

NO.9 H

FREQUENCY TRANSLATION DETECTOR

LOG NETWORKS

9S

9N

.TO COMMUTATORS

FREQUENCY TRANSLATION CORRECTOR

VOLTAGE-TO- FREQUENCY CONVERTER

DECADE COUNTER

DIGITAL DISPLAY

713

FIG.12 SIMPLIFIED BLOCK DIAGRAM OF ANALYZER

dB per octave; these slopes were chosen specifically to approxi-

mate the inverse of the idealized spread-of-masking curve.*

The output of each filter is detected and fed to a "speech" and

a noise integrator. The integrators are of the operational-

amplifier/feedback-capacitor type, and provide true time inte-

gration of the unipolar detector output signals. The "integrate/

hold" and "clear" functions are controlled by reed relays mounted

directly on each integrator printed circuit board.

The "speech" integrators are enabled during the one-second in-

terval in which the "speech" signal is produced by the signal

generator. At the end of this one-second period, integration

ceases and the final value of the integral is held. The value

stored in the "speech" integrators, it should be noted, is

really the integral of the detected "speech" plus system noise.

During the following one-second interval, the noise integrators

for the first five bands are enabled. Since no "speech" is

transmitted at this time, the final voltage stored in these

integrators is proportional to the noise alone. The frequency

of the AGC tone during this interval, the reader will recall,

is 3000 cps; this frequency is greatly attenuated by all of

the filters in bands 1 through 5, so that the presence of the

tone does not affect the measurements made in these bands.

At the end of this second one-second interval, the band 1-5

noise integrators are switched to the "hold" mode, and in the

* The spread-of-masking function is dependent upon both the fre- quency and level of the masking noise; for the purposes of SCIM we have chosen an approximation which in our opinion, is suitable for use with typical communications systems. This idealized spread-of-masking curve falls off above the frequency of the masking noise at a rate of 18 dB per octave; the masking effect at frequencies below that of the masking noise is considered to be negligible.

-22-

I I I

third one-second interval the noise integrators associated with

bands 6 through 9 are enabled.

After the entire 3-second sequence has been completed, the an-

alyzer enters its "calculating" mode. At this time, the outputs

of the band 1 (250-500 cps) "speech" and noise integrators are

connected, by means of a commutating switch, to two logarithmic

converters. Each log converter is a temperature-stabilized

diode network which delivers, at its output, a d.c. voltage

proportional to the logarithm of the d.c. voltage of its input.

The outputs of the two log networks are compared by a very

stable d.c. differential amplifier. The output of this ampli-

fier is proportional to (log["speech" + noise]) - (log[noise]),

which may be rewritten as

log "speech" + noise , S + N noise " log N

A diode correction network, having the transfer characteristic

eout = K los 4 (1° ln " X)

effects the transformation of log (S+N/N) into log (S/N) + 12 dB,

as required by the method for AI calculation.

The d.c. output voltage of the diode network is fed via a unity-

gain buffer amplifier to a voltage-to-frequency converter. This

device accepts a d.c. voltage at its input and outputs a pulse

train, the PRF (pulse repetition frequency) of which is directly

proportional to the applied voltage. Thus, in a fixed time, the

converter generates a number of pulses directly proportional to

the S/N ratio. These pulses are counted by a three-place decade

counter whose displays (of the two higher-order digits only)

constitute the readout display of the analyzer.

-23-

After dwelling for one second on the outputs of the band 1

"speech" and noise integrators, the commutator is advanced

one step, thereby connecting the outputs of the corresponding

band 2 integrators to the two log converters. After another

one-second dwell interval has elapsed (during which time the

decade counter adds to its previous total a number proportional

to the S/N ratio in band 2) the commutator examines the band 3

integrators, etc. When the commutator comes to rest in posi-

tion 10, the S/N ratios of all nine analyzer bands have been

totalized; the number displayed is the Speech Communication

Index (SCI), and is defined as

9 SCI - Y" [20 log (S/N)i + 12 dB]

i=l

The theory behind the Articulation Index requires that, in the

calculation of AI, all S/N ratios in excess of +18 dB be con-

sidered equal to +18 dB. The reason for this boundary condi-

tion is simply that the contribution to intelligibility of any

given band improves as the S/N ratio improves and reaches its

maximum value at an S/N ratio of +18 dB. Larger S/N ratios,

therefore, cannot further improve the intelligibility, and

should not contribute to the total AI in an amount greater

than that corresponding to a +18 dB S/N ratio.

The AI theory also requires that S/N ratios less than -12 dB

(corresponding to zero contribution to intelligibility) be

considered equal to -12 dB. The reasoning is, of course, just

the converse of the argument given above for the limiting of

S/N ratios in excess of +18 dB.

The boundary conditions described above are imposed in the

analyzer by the d.c. differential amplifier. Its circuitry is

such that its output cannot rise above +18 volts. Since this

-24-

output level has a scale factor of 1 dB/volt, an 18 volt output

corresponds to an 18 dB S+N/N [or (S/N)] ratio; after the

"addition" of 12 dB by the diode correction network, this cor-

responds to the upper limit of the required 30 dB range.

The amplifier also serves to impose the lower boundary limit;

since its output must always be > 0 volts (i.e., > 0 dB).

A complete measurement run, including three seconds of "speech"

and AGC tone transmissions and nine seconds of "calculation"

time may take as little as 11 seconds, since the first second

of calculation time can be coincident with the third second of

transmission from the signal generator.

Analyzer sync detector. The analyzer sync detection system con-

sists of two filters, centered at 600 cps and 2 kc, respectively,

whose outputs are detected and ANDed together. Whenever both

sync tones are found to be present, a relay is actuated which

advances a cam-operated rotary stepping switch. This switch,

in turn, provides sequentially the commands for the analyzer to

clear all integrators, integrate "speech," integrate noise,

begin calculation, etc.

The analyzer control system also contains a timing mechanism

for the commutator, which sequentially connects the logarithmic

attenuator inputs to the outputs of successive pairs of inte-

grators.

Prior to an actual measurement run, the frequency of the 1 kc

calibration tone is measured by means of a special discriminator

filter. The filter output is detected and compared with the

detected input to the discriminator. The difference between

these two d.c. voltages is a measure of the mismatch between

the transmitter carrier frequency and the receiver local

oscillator frequency.

-25-

A d.c. servo loop utilizes this "tuning error" voltage and sets

a motor-driven servo-potentiometer according to the degree of

tuning error (frequency shift) present. The servo-potentiometer

attenuates the d.c. voltage fed to the voltage-to-frequency

converter, and thereby directly multiplies the AI by some num-

ber < 1.00. The shape of the discriminator filter is adjusted

so that the resulting decrement in AI is in accordance with the

data in Fig. 13.

Operator requirements. Ease of operation of the SCIM, in the

field and by untrained personnel, has been given major considera-

tion; the only front-panel controls on the signal generator are

a power on/off switch, a metered level control, and a press-to-

start switch. The generator is internally calibrated so that,

once the level of the 1000 cps calibration tone is established

and the START button is pressed, the various signal sources

within the generator will be presented to the input of the com-

munication link under test in the proper sequence, for the

proper durations and at the required levels with no further

attention from the operator.

The controls of the analyzer are equally simple - a power on/off

switch, a metered level control and a manual reset switch. The

operator need only set the level of the incoming 1000 cps cali-

bration tone to 0 VU on the meter. The manual reset switch need

be used only after power is first applied to the analyzer or

when a severe signal dropout results in loss of synchronization

during a measurement run. Under normal circumstances the

analyzer will be ready for a second measurement run immediately

upon completion of the first. It will continue to display its

last calculated AI indefinitely, unless power is removed or

another run is begun. The operator may, therefore, read the

calculated AI from the digital display at his leisure.

-26-

I ro

i

LU

en

i.i

0.9

0.8

0.7

0.6

0-5 -80

AVERAGE OF THREE MEASUREMENTS

A DETECTOR OFF O DETECTOR ON

-60 -40 -20 0 20 FREQUENCY TRANSLATION IN CPS

40 60 80

FIG.13 SCI MULTIPLIER vs. FREQUENCY SHIFT

SECTION 4

TEST RESULTS

There are two criteria that can be used for evaluating the

accuracy and reliability of SCIM — (l) how well "SCI's" for

a variety of speech systems agree with AI!s calculated in

accordance with the 20-band method for calculated AI; (2) how

well the SCI's, for the systems being evaluated, predict the

intelligibility of speech as measured with typical psycho-

acoustic test methods.

SCI vs AI. Figure 14 illustrates the agreement between cal-

culated AIfs and SCI's as measured by SCIM when the S/N ratio

present in an otherwise "high-fidelity" speech system was

systematically varied. It is seen in this figure that except

for values below about 0.4, SCIM estimates the calculated AI

within + .02 of its true value; below 0.4 the disagreement

increases to approximately + .04. Thus, for at least the

system and noise conditions represented in Pig. 14, an average

of one, or perhaps two, readings of SCIM would provide an SCI

that has nearly exactly the same AI as would be found by the

standard calculation procedures.

SCI vs speech test scores. A series of psychoacoustic tests

were conducted on a wide variety of speech systems in order

to determine the relation between SCI and measured speech

intelligibility. For these tests a listening crew of eight

high school and college students were used. The speech tests,

"Modified Rhyme" (MR) were recorded at the output of the

various systems and noise conditions to be evaluated; these

recordings were then administered to the listening crew in

-28-

0.2

i ro

i

Q UJ

_J <

C/) 3

o if)

Q UJ

<

0.1

-0.1

-0.2

AVERAGE OF FIVE READINGS

MAXIMUM

-0.3 -12 -8 -4048

SIGNAL-TO-NOISE RATIO IN DB 12 20

FIG.14 SHOWING THE MAXIMUM, M I N I MUM AND AVERAGE VALUES OF THE DEVIATION OF FIVE MEASURED SCI'S FROM THE CALCULATED Al 'S AT S/N RATIOS FROM-10 DB TO + 22 DB

accordance with an experimental design that more or less ran-

domized the order in which the different systems and conditions

were tested. Two 50-word MR for each of two male talkers were

given at each test condition. The listeners wore earphones and

were seated in a soundproofed room. All noise conditions were

achieved by electrically mixing, at the output of the speech

transmission system, the speech with a white noise that had

been passed through a 4400 cps low-pass filter. The signal-

to-noise ratios (S/N) were measured on a VU meter. The peak

VU meter readings, in the absence of the masking noise, were

averaged for a sample of the test words. The speech signal

was then turned off and the level of the noise adjusted until

the VU meter had the same value as the average found for the

peaks of speech in the absence of the noise; the S/N was then

said to be 0 dB, if the noise exceeded the average speech

peaks by 5 dB, the S/N was called -5 dB, and so forth.

Five SCI's were measured with SCIM for each of the test con-

ditions. The results of the speech tests and the SCI measure-

ments are presented in Table 1, and the averages, for most of

these results, in Fig. 15.

Figure 15 shows that (except for certain of the 75 cps fre-

quency shift with peak clipping conditions) the measured SCI

values predict the test scores within + about 10 percentage

points. This amount of deviation (when leaving out some of

the 75 cps frequency shift conditions) compares reasonably

well with the range of differences between calculated AI and

measured intelligibility, as shown in previous studies given

in Figs. 5 and 6.

-30-

Condition Number

S/N, db Note 1

Frequency Shift, cps

Peak Clip- ping, dB

Low-Pass Filter Cutoff Frequency Kc - Note 2

Average of Average of Average of 2 MR Tests 2 MR Tests 4 MR Tests Talker CW Talker AH Two Talkers

9*».5 79.4 87.0

74.0 66.0 70.0

50.6 53.1 51.8

96.7 88.3 92.5 86.0 75-0 80.5

73.2 70.3 71.8

93.7 78.7 86.2

91.0 77.3 84.1

65.7 63.7 64.7

69.0 55.8 62.4

63.7 64.6 64.2

69.6 58.9 64.3

71.1 59.7 65.4

62.9 58.3 60.6

88.9 85.7 87.3

69.8 62.0 65.9

84.6 73.4 79.0

70.7 63.3 67.0

61.4 51.7 56.5

52.0 64.5 58.3

55.9 62.8 58.2

46.3 49.6 47.9

47.2 42.8 45.0

40.0 46.0 43.0

85.2 72.0 78.6

69.4 70.9 70.2

74.6 79.7 77.2

67.3 5O.0 58.7

90.6 81.7 86.2

85.4 73.4 79.4

88.3 8o.6 84.5

86.0 77.3 81.7

89.3 86.7 88.0

68.5 61.9 65.2

93.8 95.3 94.6

85.0 85.0 85.0

95.3 91.8 93.6

75.3 79.5 77.4

77.8 77.8 77.8

86.0 73.3 79.7

Average of 5 SCI Readings

Maximum Deviation of any SCI Read- ing from Average

.71 +.01

-35 +.01

.27 +.01

.84

.79 +.08 +.01

.57 +.01

.64 +.01

.44 -.03

.37 +.02

.39 +.02

.46 +.01

.47 -.03

.38 -.02

.28 +.01

.57 +.01

.65 +.03

.36 +.01

.36 +.02

.38 +.01

.34 +.04

.29 +.01

.35 +.02

.21 +.02

.25 -.01

.47 +.01

.46 +.02

• 25 +.02

.23 +.01

.90 -.14

.86 -.12

.74 +.05

.74 -.21

.63 -.14

.63 ±.20

.66 +.09

.79 +.05

• 58 -.05

■ bb -.05 .46 +.01

.32 +.01

I

H l

l

2

3 4

5 6

7 8

9 10

11

12

13 14

15 16

17

18

19

20

21

22

23 24

25 26

27

28

29

30

31

32

33 34

35 36

37 38

39 4o

+15 + 3 - 3

o

+15 +15 +15 +15 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 + 3 - 3 - 3 - 3 - 3 - 3 - 3 - 3 - 3 - 3 - 3

0

0

0

0

0

0

0

0

0

0

+ 3 + 3

0

0

o 0

o 0

+75 -75

o o

+75 +75 -75 -75

o 0

+75

-75 o 0

+75 +75 -75 -75

0

0

+75 -75

0

0

+75 +75 -75 -75

o 0

+75 -75 +75 -75

o o o o o 0

o 0

0

0

o 0

o 0

18

18

18

18

0

o o 0

0

o 18

18

18

18

0

0

o 0

0

0

18

18

18

18

18 18

5 5 5 5 3

1.5 5 5 3 1.5 3 1.5 3 1.5 3 1.5 3

1.5 3 1.5 3

1.5 3 1.5 3

1.5 3 1.5 3 1.5 3 1.5 3 1.5 3 1.5 3 1.5 1.5 3

Field Trial Number

Transmission Medium

Total Round-Robin Path Length, Miles Terminals

Average of One List Average of Each 20 SCI 6 Talkers Readings

Maximum Deviation of any SCI Read- ing from Average

IA

IB

IIB

Microwave

HF SSB Radio

HP SSB Radio

24

5300 5300

Andrews AFB « > Brandeywine, Md 96.9

Andrews AFB < > Azores 77.5 Andrews AFB< > Azores 76.0

.97

.69

. 64

+.01

±•19

+.20

Note 1 - All noises were white noise, 4400 cps low-pass filtered (-36 dB/octave) except those at a 0 dB S/N ratio. The masking source at 0 dB ratios was speech (continuous discourse by a single talker [CW. ]).

Note 2 - Low-pass filters had -36 dB/octave slopes above cutoff frequency.

I

i

o LU cr cr o o

Q cr o

LU

UJ

>- X cr a LÜ

o

-z. o cr ÜJ CL

100

80

60

40

20

O

o

2 o

D O _

1 "l

▲

O ▲

äp O

o

Ö

A

o O LABORATORY SIMULATIONS

A ANDREWS AFB AZORES (5300 MILES TOTAL) HF SSB RADIO ROUND-ROBIN

D ANDREWS AFB-BRANDYWINE, M D. ( 24 Ml LES TOTAL) MICROWAVE ROUND- ROBIN

▲ LABORATORY SIMULATIONS INCLUDING ± 75 CPS FREQUENCY SHIFT AND I8DB PEAK CLIPPING

0 0.10 0.20 0.30 0.40 0.50 0.60 0.70

SPEECH COMMUNICATION INDEX (SCI)

0.80 0.90 .00

FIG.15 SHOWING PERCENT MR TEST SCORES AS A FUNCTION OF SCI FOR LABORATORY SPEECH SYSTEMS, AF H-F SSB, AND MICROWAVE RADIO SYSTEMS. (SEE TABLE 1)

It is not possible to determine, from these tests, the source

of the "error" causing this range of differences -- whether

it lies with SCIM or with the speech tests. The speech tests

themselves have an unreliability, for the number here admin-

istered, not much smaller than the range of differences found

between SCI and measured intelligibility.

However, SCI can be expected to represent only an approxima-

tion to what the true capability of a system is for trans-

mitting speech. For one thing, the use of only nine filters

in the analysis set does not provide the fineness of spectrum

analysis that would be necessary to insure extreme accuracy,

and the other characteristics of the SCIM system (designed

to correct or account for, in addition to frequency-amplitude

irregularities in a transmission system, spread-of-masking

effects, frequency shift, and amplitude distortion) represent

somewhat idealized engineering averages for evaluating the

effects of the variables involved.

All in all, it would appear that, except for certain condi-

tions involving frequency shifts in excess of 60 cps or so

when combined with peak clipping, SCIM gives an index measure-

ment that can be used for evaluating a wide variety of speech

systems and listening conditions that is nearly as "accurate"

as calculated AI and/or measured percent of several MR speech

tests administered to a crew of 8 trained listeners.

-33-

SECTION 5

FUTURE DEVELOPMENTS

We are planning to make certain additions to SCIM in order to

make it a more general and useful instrument:

1. A provision will be provided in future instruments

for a digital printout so that a printed record of the SCI

value, time of day, date, etc., will be provided either with,

or remotely from, the SCIM analyzer unit.

2. Acoustic couplers ("artificial heads") will be

developed for use with SCIM. The "heads" will be such that

a microphone or headset, earphone or loudspeaker can be

attached to or placed near the head as would be the case

when the system was being used by a person. The "head" at

the transmitter end of the system would provide the SCIM

signal acoustically to the microphone or mouthpiece of the

system being tested and the "head" at the receiving end

would pick up by a microphone in an artificial ear on the

"head" the received signal for analysis purposes. When the

artificial heads are used, the SCIM will, of course, measure

the total system ambient noise conditions at both the micro-

phone and earphone or loudspeaker positions, as well as the

characteristics of the microphone and earphone.

3. Provisions will be provided so that SCIM can be

used either in its present form, that is, supplying the input

and output electrically to the system and/or acoustically;

thus one can measure the performance of the system, including

the microphone, earphone and ambient noise conditions inde-

pendently of the electrical transmission and noise character-

istics of the system.

-3*-

4. Certain modifications are being contemplated that will

permit SCIM to operate in time in a more automatic fashion than

now possible.

-35-

REFERENCES

1. French, N. R. and J. C. Steinberg. "Factors Governing the Intelligibility of Speech Sounds/' J. Acoust. Soc. Am. 1£, 90-119 (1947).

2. Kryter, K. D., "Methods for the Calculation and Use of the Articulation Index/' J. Acoust. Soc. Am. ^4, I689- 1697 (1962).

3. "American Standard Method for Measurement of Monosyllabic Word Intelligibility, S3.2-I96O," American Standards Association, New York, New York.

4. "Psychoacoustic Speech Tests: A Modified Rhyme Test," Tech. Documentary Report No. ESD-TDR-63-403, Decision Sciences Lab., ESD, AF Systems Command, L. G. Hanscom Field, Bedford, Mass., Project No. 7684, prepared by Bolt Beranek and Newman Inc. under Contract No. AF 19(628)-382 (1963).

5. Dunn, H. K. and S. D. White, "Statistical Measurements on Conversational Speech," J. Acoust. Soc. Am. 11_, 278- 288 (1940).

6. Carter, N. L. and K. D. Kryter, "Masking of Pure Tones and Speech," J. Auditory Research 2, 66-98 (1962).

7. Wathen-Dunn, W. and D. W. Lipke, "On the Power Gained by Clipping Speech," J. Acoust. Soc. Am. 30, 36-40 (1958).

8. Miller, G. A.. "The Masking of Speech," Psychol. Bull. 44, 105-129 (1947).

9. Egan, J. P. and F. M. Wiener, "On the Intelligibility of Bands of Speech in Noise," J. Acoust. Soc. Am. 18, 435- 441 (1946). —

-36-

Security Classification

DOCUMENT CONTROL DATA • R&D (Security classification ol title, body ot abstract and indexing annotation mull be entered when the overall report ia classified)

1 QRIGINATIN G ACTIVITY (Corporate author)

Bolt Beranek and Newman Incorporated. Cambridge, Massachusetts

2« REPORT SECURITY C L ASSI FIC A TION

Unclassified 26 GROUP GROl

n/a

3 REPORT TITLE

SCIM— A Meter for Measuring the Performance of Speech Communication Systems

4 DESCRIPTIVE NOTES (Type ol report end incluaive dates)

Interim Report 5 AUTHORfSJ (Lett name, first name, initial)

Kryter, Karl D., Ball, Jay H.

6 REPORT DATE

October 1964 7# TOTAL NO. OF PAGE«

38 7b. NO. OF REPS

9

8a CONTRACT OR GRANT NO.

AF19(628)-2463 b. PROJECT NO.

2808

• a. ORIGINATOR'S REPORT NUMBERfS)

ESD-TDR-64-674

• b. OTHER REPORT NOf5j (A ny other numb»re that ma irtj this report

signed

BBN Report No. 1166

10. AVAILABILITY/LIMITATION NOTICES

Qualified requesters may obtain from DDC. Copies available from OTS

11. SUPPLEMENTARY NOTES

None

12. SPONSORING MILITARY ACTIVITY Decision Sciences Laboratory,Electronic Systems Division, Air Force Systems Command, USAF , L. G. Hanscom Field, Bedford, Mass,, 0I731

13 ABSTRACT TWQ major tneories have emerged that attempt to relate in a quantitative way

the physical characteristics of a speech communication system and the intelligibility of speech as perceived by a crew of trained listeners. The more widely used theory is the basis for the "Articulation Index" (Al) formulated in I947 by N. R. French and J. C. Steinberg of the Bell Telephone Laboratories. This the- ory holds that the signal (speech) -to-noise ratio (in 20 narrow frequency bands) of a communication system will, when properly weighted and summed, provide an Al value that is directly related to the intelligibility of speech heard over that system. A second theoretical approach to this problem is embodied in the Pattern Correspondence Index (PCI) machine proposed by J. C. R. Licklider in I956. This instrument determines the correlation in the frequency and time domains between the speech input to a communi- cation system and the speech output of that system. The subject of the present paper is an electronic device called SCIM (Speech Communication Index Meter) that is designed pri- marily in terms of the Articulation Index theory; however, SCIM accommodates additional factors affecting speech that are not involved in the original procedures for calculating Al. The SCIM signal generator generates a 3-second signal burst, which is fed to the electrical input of the system under test; within 12 seconds the SCIM analyzer, located at the receiving station, calculates and displays an index, ranging from .00 to .99, which reflects the

ability of that system to transmit intelligible speech.

DD FORM 1 JAN 64 1473 -37-

Security Classification

Security Classification u.

KEY WORDS LINK A LINK B

ROLE

LINK C

Psychoacoustics Speech Speech Transmission Human Engineering

INSTRUCTIONS

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-38- Security Classification